How can we determine the coastal current of Fréjus?

1. Where is Fréjus?

Fréjus is an ancient town that is situated in the French Riviera, in the Var department of southeastern France. Nestled between the Mediterranean Sea and the Esterel Mountains, it offers a mix of natural splendor and richness in history.

It enjoys a mild climate; winters are gentle and summers are warm and sunny, hence in demand among tourists from every part of the world. Fréjus coastline is made up of sandy beaches dotted with clear blue waters and small coves-a perfect setting for relaxation and water-based activities.

Geographic area in which Fréjus is situated, it borders the Golfe de Saint - Tropez, a relatively big gulf along this coast, which has mostly quiet waters because of shelter given by capes and islands around it. The coastal waters around Fréjus are part of the Mediterranean Sea ecosystem, which, with its abundant habitat, presents marine organisms such as colorful fish, seagrass beds, and other invertebrates.

Historically, Fréjus has been an important port city. It boasts a long-standing cultural heritage with well-preserved Roman ruins, which include the Amphitheater of Fréjus dating back to the 1st century A.D. The architectural aspect of the town speaks to a combination of Roman, medieval, and modern aspects, further enhancing its appeal.

2. What is the situation of the coastal currents near Fréjus?

The coastal currents around Fréjus are governed by several factors. Therefore, the large-scale circulation of the Mediterranean Sea plays an important role. The latter has a complex system of currents, which are basically driven by differences in water density, caused mainly by variations in temperature and salinity.

The local wind patterns have a great influence on the coastal currents, too. The Mistral is a strong and dry north-westerly wind that blows over the Mediterranean, causing surface water to move away from the coast and resulting in upwelling currents. These upwelling currents bring nutrient-rich waters from deeper layers upward, which has implications for the local marine ecosystem.

Tidal forces also contribute to the coastal current dynamics near Fréjus. Though the Mediterranean Sea has a rather small tidal range compared with other oceans, the tides do interact with the local topography and bathymetry along the coastline, which in turn modifies the direction and speed of the coastal currents.

3. How to observe the coastal water flow of Fréjus?

Surface Drift Buoy Method

The surface drift buoy method involves deploying buoys on the water surface. These buoys are equipped with tracking devices, such as GPS. As the buoy drifts with the water current, the tracking device records its position over time. By analyzing the movement of the buoy, scientists can estimate the direction and speed of the surface current. However, this method provides information about the surface layer of the water column and is subject to wind-induced drift, which may introduce errors if not appropriately corrected.

Anchored Ship Method

In the anchored ship technique, the ship remains stationary but equipped with current-measuring instruments. Onboard, the ship then anchors at the desired location while instruments, like current meters, are dropped into the water at different depths. The devices used to measure currents determine the speed and direction of the flow of currents where they exist. This technique can yield high resolution of the vertical profile of the current, but it is confined to the position of the anchored ship and may be influenced by the presence of the ship, which interferes with the natural flow of the water.

Acoustic Doppler Current Profiler (ADCP) Method

In recent years, the ADCP current profiler method has been in great demand. In fact, ADCPs use reflected sound waves in measuring the velocity of water current at different depth points. In this sense, it will enable the signature detection of the current profile from the surface to the bed with no physical contact within water depth. It is non-intrusive, covers a very large area, and can record continuously, making it far more efficient than the previous two.

4. How Do ADCPs Using the Doppler Principle Work?

The basic principle of operation for ADCPs is the Doppler effect. A sound wave, emitted by an ADCP into the water, travels through the water and bounces off small particles suspended in the water. Examples are plankton, sediment, or bubbles, all moving with the water current.

The frequency of the sound wave returning to the ADCP from the sediment particles is different from the frequency of the wave that was emitted. This frequency shift, called the Doppler shift, is proportional to the velocity of the particles and hence the water current, relative to the ADCP. By measuring the Doppler shift of the reflected sound waves from different depths, the ADCP calculates the velocity of the water current at that depth.

Most ADCPs possess multiple acoustic beams that send signals at various angles, normally four or more. This multi-beam setup is what really enables the ADCP flow meter to make very accurate determinations of the three-dimensional components of the water current-again, the east-west, north-south, and vertical velocities.

5. What's needed for high-quality measurement of Fréjus coastal currents?

For high - quality measurement of the coastal currents near Fréjus, a number of characteristics in the measurement equipment is desirable.

Material Reliability

Reliable materials must be used in the making of the equipment since it will be operating in the marine environment where saltwater renders it corrosive. Titanium alloy is very appropriate for the making of ADCPs casing. This is because the titanium alloy has several advantages. It has a high strength-to-weight ratio, thus allowing it not to be overweight yet withstand the pressure of the water column at any given depth. The material is highly resistant to corrosion even in harsh saltwater conditions of the Mediterranean Sea. Due to this corrosion resistance, the ADCP current meter is able to have long-term durability and thus reduces frequently needed maintenance or replacement.

Small in Size, Light in Weight, and Low Power Consumption

The instrument should be small in size and lightweight. This facilitates easy deployment and handling, especially where large-scale measurements have to be made with many devices. Apart from that, low power consumption also ensures a long operational cycle without a fast change of batteries or hookups to extensive and complex power. This aspect is important in case of autonomous measurement systems: such applications may occur when operating over a large coastal zone or when implementing a measurement campaign with a long monitoring period.

Low Cost

A low-cost device is also very much welcome. This will enable a sufficiently large number of ADCPs to be deployed for adequate coverage of the Fréjus coastal area. By doing this, the cost will be within reach of more research institutions and monitoring programs interested in the understanding of the coastal current dynamics.

6. Comment Choisir le Matériel Adéquat de Mesure du Courant?

En fonction de l'Utilisation

• Ship-borne ADCP: These ADCPs are fixed on a moving ship. In this case, it becomes suitable for large-scale survey of coastal currents over a vast area. During any movement of the ship, while in motion, an ADCP has the capability of continuously measuring the current profile along its track.
• Bottom-mounted (Sit-on-bottom) ADCP: Those deployed on the ocean floor. The bottom-mounted or sit-on-bottom type is ideal for long-term current conditions that need to be monitored at one fixed location. In addition, this gives continued information about current velocity and direction at that very site, thus enabling the study of the local pattern and time variation of these currents.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys. They are useful for measuring currents in areas where access by ship may be difficult or for monitoring the movement of water masses over a large area. The buoys can drift with the currents, providing data on the movement of the water column as a whole.

Based on Frequency

• 600kHz ADCP: This frequency suits the measurement of currents in relatively shallow waters, typically up to a depth of about 70 m. The higher frequency allows a more detailed measurement of the current in the upper layers of the water column.
• 300kHz ADCP: Lower frequency allows deeper penetration into the water column. Suitable for water depths of about 110m. The lower frequency provides a better balance between depth penetration and measurement resolution.
• 75kHz ADCP: This low-frequency ADCP is intended for deep water and current measurements at up to 1000m. Its low frequency makes the sound waves able to travel farther in water, making it useful in current measurements at sea beds of greater depth.

There are quite a number of well-known ADCP brands available on the market. Some include Teledyne RDI, Nortek, and Sontek. In fact, for this class of people, who are in dire need of economical efficiency in high-quality solutions, a Chinese brand is highly recommended: China Sonar ​PandaADCP. This device is made of all-titanium alloy material that will guarantee excellent resistance to corrosion and a long life span. Its price-to-performance ratio is truly remarkable, hence ideal for those on a budget yet still in need of reliable data on current measurements. You can learn more about the China Sonar PandaADCP at:​ https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.